Format USB Disk as UFS
Find the disk using dmesg
dmesg
...
umass0 at uhub0 port 12 configuration 1 interface 0 "Seagate Ultra Touch HDD" rev 3.20/17.07 addr 8
umass0: using SCSI over Bulk-Only
scsibus4 at umass0: 2 targets, initiator 0
sd1 at scsibus4 targ 1 lun 0: <Seagate, Ultra Touch HDD, 1707> serial.0bc2ac1a0000NAB29XP4
sd1: 953869MB, 512 bytes/sector, 1953525167 sectors
...
Check disk allocation
fdisk sd1
Disk: sd1 geometry: 121601/255/63 [1953525167 Sectors]
Offset: 0 Signature: 0xAA55
Starting Ending LBA Info:
#: id C H S - C H S [ start: size ]
-------------------------------------------------------------------------------
0: 07 0 32 33 - 121601 57 56 [ 2048: 1953521664 ] NTFS
1: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
2: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
3: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
Allocate whole disk to OpenBSD
fdisk -e sd1
> reinit
> write
> quit
Check disk allocation
fdisk sd1
Disk: sd1 geometry: 121601/255/63 [1953525167 Sectors]
Offset: 0 Signature: 0xAA55
Starting Ending LBA Info:
#: id C H S - C H S [ start: size ]
-------------------------------------------------------------------------------
0: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
1: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
2: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
*3: A6 0 1 2 - 121601 80 62 [ 64: 1953525103 ] OpenBSD
Create a single partition of the whole Disk
disklabel -E sd1
Label editor (enter '?' for help at any prompt)
sd1> p
OpenBSD area: 64-1953525167; size: 1953525103; free: 1953525103
# size offset fstype [fsize bsize cpg]
c: 1953525167 0 unused
sd1> a
partition: [a]
offset: [64]
size: [1953525103]
FS type: [4.2BSD]
sd1*> p
OpenBSD area: 64-1953525167; size: 1953525103; free: 47
# size offset fstype [fsize bsize cpg]
a: 1953525056 64 4.2BSD 8192 65536 1
c: 1953525167 0 unused
sd1*> w
sd1> q
No label changes.
Format the disk
doas newfs sd1a
Try to mount the newly formatted disk
mount /dev/sd1a /mnt/usbdisk
cd /mnt/usbdisk
touch /mnt/usbdisk/newfile
ls /mnt/usbdisk
umount /mnt/usbdisk